Multiple local probe measuring device and method

ABSTRACT

The invention provides a local probe measuring device for effecting local measurements referring to a sample, comprising a first local probe for local measurements with respect to a sample or a reference surface, a second local probe for local measurements with respect to the sample or the reference surface, a measurement condition adjustment arrangement adapted to commonly adjust a first measurement condition of the first local probe with respect to the sample or the reference surface and a second measurement condition of the second local probe with respect to the sample or the reference surface, a detection arrangement comprising a first detection arrangement associated with the first local probe adapted to independently detect first measurement data referring to local measurements reflected by said first local probe and a second detection arrangement associated with the second local probe adapted to independently detect second measurement data referring to local measurements effected by said second local probe. Further, methods for effecting local measurements and local manipulations by means for multiple local probes are provided.

This application is a divisional U.S. application of Ser. No.10/942,875, filed Sep. 17, 2004, which is a divisional U.S. applicationof Ser. No. 10/252,363, filed Sep. 24, 2002, which is a divisional U.S.application of Ser. No. 09/399,961, filed Sep. 20, 1999, which issuedApr. 8, 2003 as U.S. Pat. No. 6,545,492.

FIELD OF THE INVENTION

The present invention concerns a multiple local probe measuring devicefor effecting local measurements referring to a sample multiple localprobe measuring method and a multiple local probe manipulation method.One application field of the novel device and the novel methods is thescanning force microscopy (SFM), also known as atomic force microscopy(AFM). However, the invention is not restricted to such an application.The basic concept is applicable to the whole range of local probetechniques developed so far, especially all other scanning probemicroscopy (SPM) techniques which require stabilization of measurementconditions, e.g. distance relations, for microscope probes, possiblycantilevers, for high-resolution measurements. Since it is hardlypossible to give a complete list of scanning probe microscope techniquesto which the invention can be applied, only some important techniquesare given: scanning tunneling microscopy (STM), magnetic forcemicroscopy (MFM), scanning near-field optical microscopy (SNOM), lateralforce microscopy (LFM), electrical field/force microscopy (EFM),magneto-tunneling microscopy and spin sensitive tunneling microscopy.

An object of the invention is to allow measurements with well definedmeasurement conditions. To this end, the invention provides for at leastone of a stabilization of measurement conditions and a calibration anddetection of measurement conditions.

In many local probe microscopy techniques, a distance of a local probewith respect to a sample or a reference surface is an essentialparameter defining the measurement conditions. Accordingly, at least oneof a stabilization, calibration, and detection of a distance associatedwith a local probe with respect to a sample or a reference surface is acentral field to which the invention can be applied. As will beexplained in more detail, the invention proposes providing a pluralityof local probes to allow at least one of a calibration, detection, andstabilization of measurement conditions for at least one local probe onthe basis of measurement effected with respect to at least one otherlocal probe. For many applications, it will be sufficient to provide twolocal probes, one of the local probes being used for at least one ofcalibration, detection, and stabilization of the measurement conditionsof the other local probe.

In the following the background and concept of the present inventionwill be exemplified with reference to the scanning force microscopytechnique on the basis of two local probes in the form of cantilevers,as commonly used for scanning force/atomic force microscopy. Accordingto the invention, there is provided a detector arrangement allowingindependent detection of first measurement data referring to localmeasurements effected by first local probe and independent detection ofsecond measurement data referring to local measurements effected by asecond local probe. In the following, it will be assumed that thisdetection arrangement is realized by a double sensor system.

The concept of the invention can easily be extended to multiple localprobe measurement devices having more than two local probes by providinga corresponding detection arrangement adapted to independently detectmeasurement data for each local probe with respect to the sample or thereference surface. Such a detection arrangement may be realized by amultiple independent sensor system. The provision of more local probesthan a first probe and a second local probe allows a further increase ofthe stability and well defined measurement conditions possiblycomprising a well defined orientation of a local probe inthree-dimensional space.

BACKGROUND OF THE INVENTION

Scanning force microscopes (SFM) were in developed in 1986 by Binnig etal. (compare: Binnig, G. et al., PhysRev Letters, 1986, Vol. 56(9), p.930-933) for imaging non-conducting surface with atomic resolution. Theyhave since become a widely used tool in the semi-conductor industry,biological research and surface science. The first SFM was basically athin metal foil acting as a cantilever, which was jammed between anSTM-tip and the sample surface. Since the cantilever was a conductingmetal, it become possible to measure the surface corrugation ofnon-conducting samples by monitoring how the foremost tip of thecantilever pointing towards the sample was deflected while moving acrossthe sample surface on the basis of a tunneling current between thecantilever and a probing tip according to the scanning tunnelingmicroscopy principle. Today, the registration of a laser's deflectionfrom the back of the cantilever on a segmented photodiode is commonlyused for this task (compare: Meyer, G. et al., Physics Letters, 1988,Vol. 53, p. 1045-1047).

Just as Binnig and Rohrer were originally interested in doing localspectroscopy on superconductors while developing the scanning tunnelingmicroscope (STM) in 1981 (compare: Binnig, G. et al., ApplPhys Letters,1982, Vol. 40, p. 178-180). The SFM was soon applied to localmeasurements of forces between different materials in vacuum, gaseousatmospheres, and in liquid. For many researches in different fields, theSFM has become an instrument for measuring local force-distance profileson the atomic and molecular scale. Measurements that have been performedrecently were concerned with ligand-receptor binding forces (compare:Florin et al., Science 1994, Vol. 264, p. 415-417), the unfolding andrefolding of proteins (compare: Rief et al., Science, 1997, Vol. 276, p.1109-1112), stretching of DNA as well as monitoring charge migration onsemiconductors and conductor/insulator surfaces (compare: Yoo, M. J., etal., Science, 1997, Vol. 276, p. 579-582).

Local measurements of forces between tip and surface suffer from thefollowing problems: 1) drift of the positioning arrangement, generally apiezo (immediately after the piezo has been extended or compressed); 2)hysteresis of the positioning arrangement or piezo; 3) mechanical drift;4) thermal drift between sensor and sample on time scales ranging fromseconds to hours; and 5) general mechanical instability resulting fromthe fact that the sensors' mechanical “feedback” on the sample istypically realized via a mechanical arm of much larger dimensions andmass than the sensor itself.

These problems can be alleviated to some degree if the force between tipand surface and, therefore, the distance between substrate and samplesurface is kept constant, for instance, by keeping the deflection angleof the cantilever constant (constant force mode). This is restricted tocases, though, where the lever (cantilever) is actually in contact withthe sample surface and the normal force on the tip is large enough so asto be well distinguishable from any background noise.

A minimal force-level in the range of a hundred pN is generally requiredto provide a stable feedback control. Many interactions, especially ofbiological molecules under physiological conditions, are in the rangewell below 100 pN down to the level of thermally induced fluctuationforces of the cantilever. Presently available instruments are notcapable of locally stabilized measurements at well-defined distancesfrom the sample in this important force range of thermally fluctuatingsensors (few pN).

Furthermore, data often need to be sampled locally over time periods ofseconds to hours. Stability problems (as enumerated above) ofinstruments available to date ultimately render such measurementsimpossible.

OVERVIEW OF THE INVENTION

One object of the invention is to provide a fast, independent, active,and in itself stable control of measurement conditions for local probemeasurements, possibly the distance between a sensor tip and a samplesurface.

Another object of the invention is to provide a way to detect thedistance between the sensor tip and the sample surface.

Another object of the invention is to provide a way to calibrate thedistance between the sensor tip and the sample surface.

According to one aspect, the invention provides a control system toachieve at least one of said objects. The basic concept behind thiscontrol system is based on the fundamental idea of appropriatemechanical and geometric scaling of feedback components for spacialstabilization of sensors used in local probe techniques.

Development on local probes in general and scanning probe instruments inparticular has lately been focused largely on the miniaturization ofprobes for measurements of very small distances, forces and energies.Problems with the stability of such instruments result from overlookingthe fact that mechanical stability of such systems is still controlledby feedback components of relatively large mass which are linked by moreor less rigid connections over long distances and which are usually madeof different materials as well. Especially scanning probe instrumentsare characterized by such connections which reach from the sensor holdervia some more or less rigid coupling to the instrument body to thescanning stage and finally the sample holder.

The basic idea behind faster and more stable feedback controls proposedhere rests on the concept of reducing the distance as well the mass ofthe mechanical coupling between the sample and sensor as much aspossible. This can be done by short-distance-low-mass closing of the“mechanical feedback loop” through rigidly coupling two or moreindependent miniature sensors for generating at least one independentforce-distance-control feedback signal. By reducing mass and length ofthe mechanical coupling in the feedback loop to the dimensions of thesensory-system intended for measurements one increases substantiallystability and with an independent distance measurement the experimentalfreedom.

One example for a multi-sensor system according to the inventionexemplifying the concept of the invention is a system having oneadditional cantilever/sensor in a scanning probe microscope (SPM),according to a preferred embodiment in a scanning force microscope (SFM)for achieving higher stability and gaining access to a new applications.The most important advantage of such a double sensor SPM (DS-SPM) is itsunique stability over unrestricted time scales, supplying a new andsound basis for measuring forces and potentials with Angstrom spatialand pN force resolution.

The multi-sensor system will be exemplified in the following on thebasis of a double-sensor system for scanning force microscopy. Theprinciples of the invention can easily be extended to multi-sensorsystem of any scanning probe microscopy technique having two or morelocal probes.

Commercial SFM exclusively use one and the same lever/sensor for twotasks: 1) to acquire sensory-data about the interactions between tip andsample surface, and 2) to control the force-distance between tip andsurface. The double-sensor system for SFM is based on a concept allowinga stabilization and optimization of local SFM measurements, according towhich the tasks 1) and 2) are split up onto two sensors sitting next toeach other, e.g., on the same substrate. As sample and substrateapproach each other, the first sensor to reach the surface will becalled the distance sensor. The second sensor to reach the surface willthen be called the interaction sensor. The sequence according to whichthe sensors approach the surface can be determined in advance.

Which of the two tips reaches the sample surface first and how muchfurther the distance between sample and surface must be reduced for thesecond lever to come into contact with the surface depends on the sizeof and the distance between the levers employed, especially ifcommercially available cantilevers are used. A typical height differencebetween cantilevers of commercially available substrates is about 5 to10 micrometers, and a typical lateral distance between the cantileversis about 100 micrometers. The height difference may be changed bytilting the short face of the substrate against the sample surface, forexample to obtain a height difference of 1 to 50, preferably 10 to 20micrometers.

The distance sensor can always be kept in contact with the surface andwill therefore continuously supply a clear force signal for the distancefeedback with Angstrom resolution. The second interaction sensor can nowbe used for independently detecting force distance profiles on, close toand also at well defined distances from the sample surface. Splittingthe tasks of distance and interaction detection between two separatesensors sitting side by side (possibly on the same substrate) thusprovides a new level of stability to scanning probe microscopes and addsimportant additional freedom for the design of measurements.

The cantilever deflection needs to be detected independently for each ofthe two levers. This can be done by two independent optical beamdeflection setups, for example. Other options would be to use anycombination of optical, interference or electrical lever detectionschemes. Especially cantilevers with integrated piezo-crystal detectorsare very promising for this task.

Conventional SFM are based on just one sensor and become locally stableonly when a feedback signal with a sufficient signal-to-noise ratio isavailable. This is only the case after the single sensor tip hascontacted the surface and a certain deflection amplitude is reached. Thelatter must at least be greater than the combined thermal anddetection-noise-amplitude of the free lever and, therefore, easily leadsto normal forces on the scale of 100 or more pN which compromise thesensitivity, the design and the general freedom of the experimentconsiderably. A well-defined distance control before the tip of thesingle lever has reached or after it has left the surface is impossiblein these conventional setups.

The double-sensor system solves these problems by means of theinteraction sensor and the distance sensor. The distance between theinteraction sensor and the sample surface can actively be controlledwith Angstrom resolution as soon as the distance sensor has made contactwith the surface. At this point, the interaction sensor is stilltypically 1 to 50 micrometers above the sample surface. The sensorysignal coming from this lever can now be used to do measurements at anydistance from the surface and in contact with the surface at normalforces determined only by the sensitivity of the detection system andthe spring constant of the cantilever used.

The interaction sensor may approach and retract from the surface whilethe distance between substrate and sample is actively controlled by thedistance sensor feedback. Any creep or drift of the piezo in heightdirection can be corrected on long as well as on short time scales, i.e.from milliseconds to hours, possibly even days. This means especiallythat it is possible to stop the approach or retract of the interactionsensor at any desired distance from the surface for any duration oftime. The lever will stay at exactly this position, i.e. the distancebetween substrate and surface will be kept at exactly this value by thedistance sensor feedback. Depending on the nature of the sample surfaceit is also possible to scan the interaction sensor in parallel to thesurface at well defined distances.

The approach according to the invention differs substantially fromapproaches of the prior art aiming to determine the distance of thelever from the sample by additional detection means based onfiber-optical interference (compare: Martin, Y., et al., J. ApplPhys.1987, Vol. 61, p. 4723 et seq) or simultaneous capacitance measurements(compare; Barret, R. C. and Quate, C. F., J. Appl.Phys., 1991, Vol. 70,p. 2725 et seq). Both known approaches immediately set specificrequirements on the sample surface or the constancy of ionic strength ofsolutions used in fluid-cell experiments. Experimental requirements musttherefore be compromised without the attainment of true distance andthus force control.

A crucial and highly important but in no way critical point of thedouble-sensor system (or multi-sensor system) according to the inventionfor atomic force microscopy and the like is a high mechanical stabilitywith respect to the sensors, which can be reached by introducing a rigidmechanical coupling between the distance feedback sensor(s) andinteraction sensor(s) on the scale of only a few hundred micrometers,generally on a scale in the order of dimensions of the local probesthemselves. As has already been indicated, in conventional SFM amechanical coupling between the sample and the sensor is realized viamore or less rigid connections between the sample, piezo, piezo-holder,instrument base and sample holder. The coupling is thus relayed over adistance of several centimeters instead of a few hundred micrometers.

Commercially available cantilever designs for SFM already typicallyoffer several levers of different lengths and spring constants.Therefore, existing supplies of cantilever substrates can be usedtogether with a proper design of a multiple detection system, preferablya multiple optical detection system. Such optical detection systems canbe integrated rather easily in nearly all commercial instruments andthus may provide a new generation of scanning probe instruments withunprecedented stability and tip/sample approach control.

The principle of the invention can be extended to any local probemeasuring device and any local probe measuring method allowing localmeasurements referring to a sample. Accordingly, the invention providesa local probe measuring device for effecting local measurementsreferring to a sample, comprising a first local probe for localmeasurements with respect to a sample or a reference surface, a secondlocal probe for local measurements with respect to the sample or thereference surface, a measurement condition adjustment arrangementadapted to commonly adjust a first measurement condition of the firstlocal probe with respect to the sample or the reference surface and asecond measurement condition of the second local probe with respect tothe sample or the reference surface, a detection arrangement comprisinga first detection arrangement associated with the first local probeadapted to independently detect first measurement data referring tolocal measurements effected by said first local probe and a seconddetection arrangement associated with the second local probe adapted toindependently detect second measurement data referring to localmeasurements effected by said second local probe.

The local probe measuring device according to the invention may comprisea controller adapted to control via said measurement conditionadjustment arrangement said first and second measurement conditions onthe basis of one of said first and second measurement data.

It is to advantage if the controller and the detection arrangement areadapted to adjust via said measurement condition adjustment arrangementat least one of said first and second measurement conditions on thebasis of one of said first and second measurement data and then toobtain the respective other said first and second measurement data forthe adjusted measurement condition.

It is proposed that said controller, said positioning arrangement andsaid detection arrangement are adapted to adjust said first measurementcondition on the basis of said first measurement data and then to obtainsaid second measurement data for the resulting second measurementcondition or to adjust said second measurement condition on the basis ofsaid second measurement data and then to obtain said first measurementdata for the resulting first measurement condition.

Said first and second measurement conditions may comprise distancerelations of the local probes with respect to the sample or thereference surface. In this case, the measurement condition adjustmentarrangement may comprise a positioning arrangement adapted to commonlyadjust said distance relations. The positioning arrangement may compriseat least one piezo-crystal.

The local probe measuring device may comprise more than two local probesadapted to effect local measurements with respect to the sample or thereference surface. The measurement conditions of this plurality of localprobes may be commonly adjusted by said measurement condition adjustmentarrangement (possibly the positioning arrangement). In the case of morethan two local probes, it is preferred that the detection arrangement beadapted to independently detect measurement data referring to localmeasurements effected by each local probe.

Preferably, there are provisions to control measurement conditions of atleast one of said local probes (two local probes or more) via saidmeasurement condition adjustment arrangement (possibly the positioningarrangement) on the basis of local measurements effected by at least oneother local probe of said local probes. According to a preferredembodiment there are provisions to control measurement conditions of oneof said local probes on the basis of local measurements effected by atleast three other local probes of said local probes. Preferably, the atleast three other local probes are arranged around said one local probe.

At least one of said local probes may be one of an atomic forcemicroscopy probe, a lateral force microscopy probe, a tunnelingmicroscopy probe, a magnetic force microscopy probe, an electric forcemicroscopy probe, a near-field optical microscopy probe and an otherlocal probe microscopy probe. It is possible that all the local probesused during one measurement are of the same probe type. For certainmeasurements, however, it may be useful if at least two of the localprobes used during one measurement are of different probe types.

According to one aspect of the invention, a local probe measuring devicefor effecting local measurements referring to a sample is provided whichcomprises: a first local probe for local measurements with respect to asample or a reference surface, a second local probe for localmeasurements with respect to the sample or the reference surface, arigid mechanical coupling between the first local probe and the secondlocal probe, a positioning arrangement adapted to commonly adjustdistance relations of the probes with respect to the sample or thereference surface to commonly adjust a first measurement condition ofthe first local probe with respect to the sample or the referencesurface and a second measurement condition of the second local probewith respect to the sample or the reference surface, a detectionarrangement comprising a first detection arrangement associated with thefirst local probe adapted to independently detect first measurement datareferring to local measurements effected by said first local probe and asecond detection arrangement associated with the second local probeadapted to independently detect second measurement data referring tolocal measurements effected by said second local probe.

According to a further aspect, the invention provides a local probemeasuring device for measuring local interactions between a local probearrangement and a sample, comprising: a first local probe adapted tointeract locally with a sample or a reference surface, a second localprobe adapted to interact locally with the sample or a referencesurface, a rigid mechanical coupling between the first local probe andthe second local probe, a positioning arrangement adapted to commonlyadjust at least one of a first distance of the first local probe withrespect to the sample or the reference surface and a first localinteraction of the first probe with the sample or the reference surfaceand at least one of a second distance of the second local probe withrespect to the sample or the reference surface and a second localinteraction of the second local probe with the sample or the referencesurface, a detection arrangement comprising a first detectionarrangement associated with the first local probe adapted to detect atleast one of the first distance and the first local interactionindependently of the second distance and the second local interactionand a second detection arrangement associated with the second localprobe adapted to detect at least one of the second distance and thesecond local interaction independently of the first distance and thefirst interaction.

Further, according to still another aspect the invention provides alocal probe measuring device for measuring local interactions between acantilever probe arrangement and a sample, comprising: a firstcantilever probe adapted to interact locally with a sample or areference surface, a second cantilever probe adapted to interact locallywith the sample or a reference surface, a rigid mechanical couplingbetween a base section of the first cantilever probe and a base sectionof the second cantilever probe, a positioning arrangement adapted tocommonly adjust a first distance of the base section of the firstcantilever probe with respect to the sample or the reference surface anda second distance of the base section of the second local probe withrespect to the sample or the reference surface, a detection arrangementcomprising a first detection arrangement associated with the firstcantilever probe adapted to independently detect at least one of a firstdeflection of the first cantilever probe and a first local interactionof the first cantilever probe with said sample or reference surface anda second detection arrangement associated with the second cantileverprobe adapted to independently detect at least one of a seconddeflection of the second cantilever probe and a second local interactionof the second cantilever probe with said sample or reference surface.

According to another aspect of the invention, a method of effectinglocal measurements referring to a sample is provided, which comprises:providing at least two local probes in a positional relation withrespect to a sample or a reference surface, said local probes preferablybeing rigidly mechanically coupled with each other, adjusting arespective measurement condition for at least one of said local probeson the basis of measurements effected with respect to at least one otherof said local probes, and effecting a measurement with respect said atleast one local probe with reference to said measurements effected withrespect to said at least one other local probe.

According to still another aspect of the invention, a method ofeffecting local manipulations referring to a sample is provided, whichcomprises: providing at least two local probes in a positional relationwith respect to a sample or a reference surface, said local probespreferably being rigidly mechanically coupled with each other, adjustinga respective manipulation condition for at least one of said localprobes on the basis of measurements effected with respect to at leastone other of said local probes, and manipulating said sample by means ofsaid at least one local probe with reference to said measurementseffected with respect to said at least one other local probe.

Features of preferred embodiments of the local probe measuring devicesand the local probe measuring and manipulation method are set forth inthe claims which are part of the disclosure of this specification.

A local probe measuring device according to the invention, a local probemeasuring method according to the invention and a local probemanipulation method according to the invention each open up options formany new applications which can hardly be foreseen at present. In thecontext of atomic force microscopy or scanning force microscopy and thelike, some new applications are the following:

-   -   1) Samples, e.g. in biological applications, can locally be        measured at effectively vanishing normal-forces between tip and        sample. This is especially interesting for measuring specific        protein interactions, e.g. ligand/receptor interactions, in a        controlled way. This is an important feature for the application        of SFM in drug screening applications.    -   2) The invention also allows for force-spectroscopy on proteins        and polymers where contact between tip and sample can be reached        at almost zero-force and thus minimal mechanical interaction        between sample and sensor. The active control of the interaction        sensor and sample surface distance opens up the possibility of        measuring e.g. unfolding potentials of proteins with very soft        levers or the unbinding of molecular adhesion bonds under        constant force in liquid environments (compare: Evans, E. and        Ritchie, K., BioPhys J., 1997, Vol 72, p. 1541-1555).    -   3) By employing levers for the interaction sensor with different        and especially with very soft or hard spring constants it is now        possible to measure interactions close as well as further away        from the sample surface at Angstrom- and pN-resolution. For        these measurements it is necessary to keep the interaction        sensor at well defined distances from the sample surface for        times which increase as the spring constant of the lever        decreases. These times can reach up to seconds for the recording        of time-series of the thermal position fluctuations of the        sensor-tip in local potentials, which can be analyzed by        correlation and spectral transforms. Using only the thermally        excited amplitudes of the cantilever, one decreases the        influence of the sensor on the sample and the danger of        dissipating energy into the sample to a minimum and thereby        reduces the danger of locally altering or even destroying the        sample.    -   4) One option for implementing lateral scanning at well defined        distances from the surfaces may be based on e.g. Si/SiN3 sample        carriers, which would allow for atomically flat reference        surfaces over which the distance sensor may be scanned laterally        at constant normal force without changing the distance between        interaction sensor and any locally decorated sample surface.    -   5) A multi-sensor system according to the invention allows a        stabilization at minimal interaction forces for attractive mode        measurements in scanning force microscopy (atomic force        microscopy).    -   6) A multi-sensor setup according to the invention can easily be        adapted to flow chamber experiments, since thermal or mechanical        disturbances can be compensated by the multi-sensor system.        Since additional optical interference sensors are not necessary,        the exchange of fluids in liquid cells becomes easier.    -   7) Instruments equipped with a multi-sensor system according to        the invention are well suited for product control or general        measurements under mechanically unstable conditions. The only        limitation results from the speed of the feedback which        compensates for these disturbances through the distance sensor        force-distance feedback.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, some embodiments of local probe measuring devices willbe explained solely as examples and with reference to the figures.Further, some examples of applications of local probe measuring devicesaccording to the invention will be given.

FIG. 1 is a schematic diagram of a local probe measuring deviceaccording to a first embodiment of the invention having two localprobes.

FIG. 2 shows a plurality of cantilever probes to be used as local probeswhich are integral parts of a cantilever substrate, two of thosecantilever probes being illuminated by laser radiation to detectdisplacements of the respective cantilever probe.

FIG. 3 shows schematically a near-field optical microscopy probe of thecantilever type, which can be used for effecting scanning near-fieldoptical microscopy (SNOM) measurements.

FIG. 4 shows three schematic diagrams illustrating examples ofmeasurements which can be effected with a local probe measuring deviceaccording to the invention of the scanning force microscopy (SFM) oratomic force microscopy (AFM) type.

FIG. 5 shows measurement results obtained with a cantilever probemeasuring device according to the invention which refer to thermalposition fluctuations of a cantilever probe for two minute distances ofthe respective cantilever probe with respect to a surface.

FIG. 6 shows further measurement results obtained with the cantileverprobe measuring device referring to thermal position fluctuations of therespective cantilever probe for so-called “soft contact” and so-called“hard contact” of the cantilever probe with the surface.

FIG. 1 shows a multiple local probe measuring device according to oneembodiment of the present invention. The local probe measuring device 10has a first local probe 12 and a second local probe 14, which arerigidly connected with each other by means of a connection part 16,possibly a substrate which is integral with the local probes 12 and 14.The local probes may be of the cantilever type. In this case, therespective local probe may comprise a beam cantilever (cf. cantilever 18in FIG. 2) or a triangular cantilever (cf. cantilever 20 in FIG. 2)having, at a free end, a tip serving to probe a sample 22. In case ofcantilever probes, the local probe measuring device may also be denotedas cantilever probe measuring device.

FIG. 1 shows schematically the general structure of a multiple localprobe measuring device according to an example independently of anyparticular local probe measuring technique. For example, the local probemeasuring device may be used for effecting atomic force microscopy (AFM)or scanning force microscopy (SFM) measurements. Other examples arelateral force microscopy (LFM) measurements, scanning tunnelingmicroscopy (STM) measurements, and scanning near-field opticalmicroscopy (SNOM) measurements, to name but a few local probetechniques. However, the local probe measuring device 10 shown in FIG. 1may as well be adapted to effect any other local probe microscopymeasurements, such as electric field/force microscopy (EFM)measurements, magnetic field/force microscopy (MFM) measurements,scanning near-field acoustic microscopy (SNAM) measurements,magneto-tunneling microscopy measurements, and the like, which require alocal probe held mechanically in the vicinity of a sample.

Referring again to FIG. 1, the device 10 comprises a bracket 24 holdingthe connecting part 16 together with the local probes 12 and 14 over thesample 22, which is arranged on a sample stage 26. The sample stage 26and, accordingly, the sample may be finely positioned with respect tothe two local probes 12 and 14 by means of a piezo transducer 28, whichis adapted to move the sample stage 26 with the sample 22 in X-, Y-, andZ-directions.

A coarse approach and positioning of the sample 22 with respect to thelocal probes 12 and 14 may be effected by means of a coarse positioningarrangement 30, possibly comprising a support plate mounted on a tripotadjustable by means of slow-motion tangent screws or/and an electricalmotor (e.g., a DC motor) coupled by means of a gear (possibly aslow-motion tangent screw or the like) with the tripot.

If one considers one of the two local probes 12 and 14, the respectivelocal probe, on the one hand, and the sample 22, on the other, arelocated at opposite ends of a mechanical linkage loop comprising theconnecting part 16, the bracket 24, the coarse positioning arrangement30, the piezo transducer 28, the sample stage 26, and any furthermechanical connection components between the mentioned parts, such asintermediate members 34, 36 between the course positioning arrangement30, the piezo transducer 28, and the sample stage 26. If a distancebetween one of the local probes and the sample is adjusted by means ofthe piezo transducer 28 and the coarse positioning arrangement 30, anymechanical instability of the mentioned mechanical loop and any piezodrift of the piezo transducer will cause variations of the adjusteddistance between the respective local probe and the sample.

According to a prior-art approach, one could use detection resultsobtained for the respective probe by means of a detector (detector 40for probe 12, and detector 42 for probe 14) to stabilize the distancefor the respective (the same) probe. For example, one could adjust thedistance between probe 12 and sample 22 by means of the coarsepositioning arrangement 30 and the piezo transducer 28 and use thedetection result obtained by the detector 40 for this probe to controlthe distance of this probe (probe 12) with respect to the sample 22 bymeans of a feedback control loop 44 comprising the piezo transducer 28,the probe 12, the probe detector 40, and a feedback transformer 46. Thefeedback transformer 46 may comprise a high-voltage amplifierarrangement adapted to translate a control variable into a correspondingdriving voltage sent to the piezo transducer 28.

This approach works quite well if the detection signal received for therespective-local probe (in the example, probe 12) is strong enough withrespect to noise. Even if the signal is strong enough to obtain thedesired feedback control of the distance between the local probe and thesample, this approach has the advantage that there may be severalinteractions between the sample and the local probe, which mightcontribute to the detection signal. In the case of an atomic forcemicroscopy probe (possibly of the cantilever type), for example, theforces between the probe tip and the surface of the sample may resultfrom a multitude of interactions so that the central contrast mechanismon which the high resolution of atomic force microscopy is based willoften become distorted.

The explained prior-art approach of controlling the distance of ameasuring probe will completely fail if the detection signal receivedfor the local probe is too small with respect to noise so that there isno signal on which the feedback control can be based.

In realization of an approach according to the present invention, themultiple local probe measuring device 10 has a plurality of localprobes, namely the two local probes 12 and 14, with the detectionresults obtained for one of the local probes being used for the feedbackcontrol of the distances of both local probes with respect to the sampleand the desired measurement information is obtained via the other localprobe. The desired measurement information may comprise, for example,information with respect to the probe interaction versus distancebetween the respective probe and the sample surface.

In the diagram of FIG. 1, probe 12 is used for the feedback control ofthe distances of both probes 12 and 14 with respect to the samplesurface and the detection results of probe 14 are evaluated to obtainthe desired measurement information. As shown in FIG. 1, the probes 12and 14 may have different distances with respect to the sample. In thesituation shown in FIG. 1, the probe 14 has a larger distance withrespect to the sample than probe 12. Accordingly, any interactionbetween the probes and the sample, which decreases with the distances,will give a stronger detection signal for probe 12 than for probe 14.For example, an interaction between sample 22 and probe 14 or anotherinfluence on probe 14 which gives a rise to a detection signal which isnot strong enough for a feedback control of the distance, may give riseto a much higher detection signal for the other probe (probe 12) andmay, accordingly, enable a feedback control of the distances.

Assuming that the local probes 12 and 14 are cantilever probes adaptedto atomic force microscopy, FIG. 1 shows a situation in which probe 12contacts the sample surface with relatively high reaction force (forexample, 100 pN), giving rise to a corresponding deflection of thecantilever and a detection signal sufficiently strong to enable feedbackcontrol, whereas the other local probe is located well above the samplesurface and interacts with the sample only weakly, so that only a weakdeflection or weak deflections of the cantilever of probe 14corresponding to interaction forces of a few pN may take place, whichgive rise to only weak detection signals possibly not sufficientlystrong for enabling a feedback control of the distance. The interactionforces may be in the range of thermally induced fluctuation forces onthe cantilever.

According to the approach of the invention, measurements of such weakinteractions on a local probe are enabled under stabilized measurementconditions even for a long time, since the feedback control is based onthe detection signals obtained for the other local probe which stronglyinteracts with the sample. Any piezo drift or mechanical or thermalinstability of the mechanical connection loop between the probes 12 and14, on the one hand, and the sample 22 on the other hand may becompensated by the feedback control stabilization of both local probeswith respect to the sample obtained on the basis of the detection signalobtained for one local probe. Accordingly, the other local probe (probe14) has stabilized measurement conditions (i.e., a stabilized distancewith respect to the sample) to allow long-time measurements at highstability. Accordingly, even thermal fluctuations of the position(possibly deflection) of the local probe 14 may be analysed.

Referring again to the connection part 16 rigidly connecting the twolocal probes 12 and 14, the following should be mentioned. For highrequirements with respect to the stabilization, the connection partshould be characterized by a coupling path between the two local probes,which has a path length in the order of magnitude of dimensions of thelocal probes themselves. Further, the rigidity of the mechanicalcoupling effected by the connection part should be such that it hasassociated a resonance frequency well above a limiting frequencyassociated with the detectors 40 and 42, so that any vibrations of thetwo local probes with respect to each other induced by vibrations of theconnection part will not be detected. Further, if a resonance frequencycan be associated with the local probes themselves, it is preferred thatthe resonance frequency associated with the connection part lies wellabove the resonance frequencies of the local probes.

Evidently, there still may occur several interactions between the localprobe used for distance stabilization (probe 12 in FIG. 1) and sample22. Often, however, there may be an interaction regime in which one ofthose interactions dominates the other interactions so that, inpractice, there will be no significant distortion of the contrastmechanism relevant to the resolution of local probe microscopy. In anycase, one might locate the sample in such a way with respect to thelocal probes so that only the local probe used for obtaining themeasurement information interacts with the sample and the other localprobe, which is used for the feedback control of the distances,interacts with a reference surface. In this case, there will be awell-defined interaction between the local probe used for the feedbackcontrol (in the following also referred to as “distance sensor”) and thereference surface. Accordingly, the measurement condition for the otherlocal probe (in the following also referred to as “interaction sensor”)may be defined with reference to the detection results obtained for thedistance sensor, allowing stabilized measurements at well-defineddistances from the sample, for example, to obtain force-distanceprofiles without any distortions of the “distance axis” because ofmultiple interactions between the distance sensor and the sample.

The distance between the two local probes 12 and 14 in a heightdirection associated with the sample (in FIG. 1, the Z-direction) may beadjustable. In case of cantilever probes used in atomic forcemicroscopy, a distance (height) of about 10 to 20 μm may be appropriate.However, also other height differences (for example, about 1 to 100 μm)may be appropriate depending on the sample and the measurementsituation. The height difference may be adjustable, for example, bytilting a short face of a substrate integral with the cantilever probesagainst the sample surface.

Referring again to FIG. 1, the local probe measuring device 10 comprisesa controller 48 adapted to control the piezo transducer via the feedbacktransformer 46. For example, the controller 48 sets a setpoint value (adesired value) representing a setpoint distance of local probe 12 withrespect to the sample. The feedback control loop regulates theZ-position of the sample stage via the piezo transducer 28 so that adeviation between an actual value and the setpoint value approacheszero. The control characteristics of the feedback control loop may beselected as desired and may comprise at least one of an integral,differential, and proportional control characteristic.

The feedback transformer 46 may, at least in part, be realized by thecontroller 48 itself. This applies in particular when the controller isimplemented on the basis of a digital processor. However, the feedbacktransformer 46 and the feedback control loop 44 may as well beimplemented on the basis of analog electronic components. With respectto the setting of the setpoint value, the controller may even beimplemented by means of a simple potentiometer or the like. Of course, adigital implementation of the controller and of the feedback controlloop according to the state of the art is preferred.

The control scheme described so far may be supplemented by a secondfeedback control loop on the basis of measurement data referring tolocal measurements effected by the local probe serving as “interactionsensor”, i.e. the “other local probe”. This second feedback control loopshould be characterized by a time constant shorter than a time constantcharacterizing the first control loop on the basis of measurement datareferring to local measurements effected by the local probe serving as“distance sensor”. One can think of the two feedback control loops asfeedback control sub-loops of an overall feedback control loop.According to this supplemented control scheme, the feedback control onthe basis of measurement data referring to the “distance sensor” mayprovide for a compensation of thermal drift or other driftscharacterized by a rather long time constant, whereas the additionalfeedback control loop on the basis of measurement data referring to the“interaction sensor” may allow a detection of fine structures of thesample, i.e. topological details of the sample surface.

The controller 48 receives measurement data referring to the localmeasurements effected by the two local probes 12 and 14 from thedetectors 40 and 42. The received measurement data can be evaluated withrespect to the control of the piezo transducer, for example, todetermine the appropriate setpoint value. Further, the receivedmeasurement data can be processed by the controller to obtain thedesired measurement information. This information, as explained, isprimarily contained in the measurement data referring to local probe 14serving as an interaction sensor. However, also the measurement datareferring to local probe 12 serving as a distance sensor may beevaluated to obtain the desired measurement information. Thismeasurement information or/and the raw data obtained from the detectors40 and 42 may be stored in any storage device and may be displayed on amonitor or printed via a graphics printers or the like.

So far, only a control of the distances of the local probes 12 and 14with respect to the sample 22 has been considered. With this control, arelative movement of the local probes 12 and 14 with respect to thesample 22 in the Z-direction can be effected, for example, to obtainforce-distance profiles. Further, the controller 48 is adapted tocontrol the piezo transducer 28 to effect a lateral scanning or samplingof the sample 22 in the X- and Y-directions. With this lateral scanning,a topographic imaging of the sample by the probes 12 and 14 may beeffected. In case that the sample that has a planar surface or anychanges of the sample height occur over lateral distances larger thanthe lateral distance between the two local probes 12 and 14, theprovision of an additional distance sensor allows easy scanning of thesample by the interaction sensor in a “distance mode” or “lift mode” inwhich the respective distance between the interaction sensor and thesample surface can be maintained at high stability. This “distance” modemay even be used in case of a sample having height variations withlateral dimensions in the order of the lateral distance between the twoprobes. In such a case, one can, in a first step, measure the topographyof the sample by means of the distance sensor. After this first passacross the sample surface, the distance of the interaction sensor 14 canbe controlled at high stability on the basis of the measurement dataobtained during the first pass, so that in a second pass across thesample surface, the interaction between the sample surface and theinteraction sensor can be measured for a defined distance between theinteraction sensor and the sample surface. Generally, the stabilizationof distance relations or generally of measurement conditions, on thebasis of measurement data obtained for an additional local probe can beused advantageously for any measurement scheme of the respective localprobe microscopy technique.

A central requirement for the measurement scheme according to theinvention described above, is independent detection of measurement datafor the distance sensor and the interaction sensor. If there is aplurality of distance sensors, for example, three or more sensorsarranged around one interaction sensor, independent detection for allsensors is desired. However, there might be cases where independentdetection for the distance sensors or a plurality of distance sensors,on the one hand, and the interaction sensor or a plurality ofinteraction sensors, on the other, is sufficient for certain measurementsituations.

An independent detection of measurement data for a plurality of localprobes can be obtained for all detection principles used in the contextof local probe microscopy. In the case of deflecting cantilever probes,for example, one might use piezo-electrical cantilevers, each of whichproduces its own electrical deflection signals used for the feedbackcontrol or measurements of interactions with the sample, respectively.

Further, independent optical detection of cantilever deflections ispossible. FIG. 2 shows a cantilever substrate having a plurality ofcantilevers. There are two laser beams 50 and 52, one being directedagainst the back of the triangular cantilever 20 and the other beingdirected against the back of the neighboring triangular cantilever 54.For example, cantilever 20 may serve as a distance sensor and cantilever54 as an interaction sensor.

The laser beams are reflected by the cantilever backs onto a respectiveposition-sensitive sensor, for example, a segmented photo-diode, givingrise to deflection signals representing the deflection of the respectivecantilever. Since both cantilevers have associated their own laser beamand their own position-sensitive detector, the deflections of bothcantilevers may be detected independently of each other.

Besides the possible configurations mentioned so far, many otherconfigurations are possible. For example, it is not necessary that thedistance sensor and the interaction sensor are of the same local probemicroscopy type. For many measurement situations, it will be appropriateto use local probe microscopy probes of the same probe type for thedistance sensor and the interaction sensor, for example, two atomicforce microscopy probes or two scanning tunneling microscopy probes andthe like. In other situations, it might be appropriate to use differentlocal probe microscopy probes for the distance sensor and theinteraction sensor, for example, a local probe of the scanning tunnelingmicroscopy type for the distance sensor and a local probe of the atomicforce microscopy or scanning force microscopy type for the interactionsensor. Correspondingly, the same applies to cases where a plurality ofinteraction sensors or/and a plurality of distance sensors are used.

Further, the detection principle according to which data representingthe local measurement effected by the respective local probe aredetected may be different or may be the same. Generally, any detectionprinciple which appears appropriate with respect to the type of therespective local probe can be chosen to obtain measurement datarepresenting the local measurements effected by the respective localprobe. In case of the cantilever probes shown in FIG. 2, two cantileversare simultaneously probed by laser beams to measure the deflection ofthe respective cantilever. Other detection principles to measurecantilever deflections are known and need not be enumerated here. Incase of local probes of the scanning tunneling microscopy type, onewould measure a tunneling current resulting from a quantum mechanicalinteraction between a probe tip and the sample.

In case of local probes of the scanning near-field optical microscopytype, an optical far-field resulting from a near-field illumination of asample through a sub-wavelength sized aperture has to be detected toobtain a resolution beyond the Abb6 diffraction limit. Such ameasurement situation is shown in FIG. 3. A cantilever 100 serving as alocal probe has an aperture 102, which is illuminated via an opticalfiber 104 by optical radiation having a longer wavelength than the sizeof the aperture. An optical near-field on the other side of the endportion of the cantilever interacts with the sample 106 and gives riseto a far field which can be detected by a detector 108. The height ofthe aperture end portion of the cantilever 100 over the sample 106 can,according to the principles of the invention explained above, becontrolled on the basis of interactions of another local probe,preferably another cantilever probe serving as a distance sensor. Thecantilever 100, or even only the aperture 102, may be regarded as aninteraction sensor in the sense of the principles of the inventionexplained above, irrespective of the fact that in case of a measurementsituation as shown in FIG. 3, there no detection of data referring tothe interaction sensor per se, but a detection of data referring to aninteraction between the interaction sensor (the aperture 102), anoptical field (generally an interaction field), and a sample, with nodirect interaction between the detector and the interaction sensor.

Other interaction fields influencing or bringing about an interactionbetween a local probe and a sample comprise electrostatic fields,magnetic fields, and electromagnetic fields. Further, an interactionbetween a local probe and a sample may be influenced or even broughtabout by interaction media (for example, fluids, gases, gas mixture orliquids) which interact with the sample or/and with the respective localprobe. To hold such an interaction medium, the device shown in FIG. 1has a medium receptacle symbolized by broken line 110. Dependent on thesample, the interaction medium may be simply water or air.

The range of possible measurements which can be effected by using alocal probe measuring device having a plurality of local probes can beextended significantly if probes are used which are functionalized withrespect to the sample. For example, it is possible to bind sensingmolecules to an AFM tip or to colloidal fields attached to an AFMcantilever. The molecules bound to the AFM probe can then be used aschemical sensors to detect forces between molecules on the tip andtarget molecules on a surface. This allows extremely high-sensitivitychemical sensing. As with chemically modified probes, one may tailor AFMprobes to sense specific biological reactions. One could, for example,the binding forces of individual ligand-receptor pairs. To this end, anAFM probe tip may be coated with receptor molecules. Another example isthe measurement of interaction forces between complementary DNA strains.To this end, one could bind DNA to a sample surface, on the one hand,and to a spherical probe attached to an AFM cantilever, on the other.There are many other examples of possible applications. Reference is hadto the numerous literature about scanning probe microscopy and thedifferent probing technologies developed so far. For all measurementsituations in which the measurement results depend on the distance ofthe respective local probe of a sample, the principles of the presentinvention which allow independent measurement of the distance can beused advantageously. More generally: in any local probing technique inwhich well-defined local measurement conditions are desirable, theprinciples of the present invention which allow stabilization of theselocal measurement conditions on the basis of measurement data referringto local measurements effected by at least one other local probe can beused advantageously. If these local measurement conditions refer notonly to the distance or to other parameters, the additional local probecan be denoted as measurement condition sensor instead of distancesensor.

In the following, some specific examples of preferred applications of alocal probe measuring device of the atomic force microscopy typeaccording to the invention will be given with reference to FIGS. 4 to 6.It is assumed that cantilever probes are used as local probes.

FIGS. 4 a) and b) show schematically examples of force spectroscopymeasurements. FIG. 4 a 1, in the upper of the diagram, shows aforce-distance profile which can be obtained if a cantilever substrate200 is moved towards a sample surface 202 at a defined velocity untilthe probe tip 204 contacts the surface 202 and is retracted thereafter.During this positioning cycle, the force detected by the cantilever isrecorded as a function of the distance between the substrate 202 and thesurface. This distance can, according to the invention, be calibrated ormeasured by means of an additional cantilever serving as a distancesensor, since the height distance between the sensors can be measuredwith high accuracy, for example in advance of the spectroscopicmeasurements. Accordingly, the cantilever 206 having the tip 204 shownin FIG. 4 can be denoted as an interaction cantilever.

The shape of the force-distance profile shown in the diagram can beexplained as follows. Near the sample surface there is a potentialresulting from the interaction between the sample and the cantilevertip. The force sensed by the cantilever is given by the gradient of thispotential as a function of the distance between the sample surface andthe tip. Near the sample, the tip jumps into contact with the sample ifthe potential has a gradient larger than the elastic force constant ofthe cantilever. As soon as there is a hard contact between the tip andthe sample surface, the force on the cantilever is primarily governed bythe spring constant of the cantilever.

During the retraction of the substrate with the cantilever, there aregenerally larger forces acting on the tip, which give rise to thehysteresis shown in the force-distance profile.

In the lower part of FIG. 4 a), the gap between the tip and the surfaceis shown as a function of the distance between the substrate and thesurface.

FIG. 4 b) shows a schematic example of an intra-molecular forcespectroscopy measurement result. During intra-molecular forcespectroscopy measurements, molecules such as DNA strains, proteins, andso on, are stretched between a surface and an atomic force tip. Thetension forces acting on the cantilever are recorded as a function ofthe distance. According to the invention, the distance can be measuredor calibrated at high precision on the basis of deflection measurementsrelating to an additional cantilever, the distance cantilever. Anotherexample, are intermolecular force spectroscopy measurements which can beused, for example, for the measurement of adhesion forces between cells,binding forces between specific molecules, for example, ligand-receptorcombinations.

FIG. 4 c) schematically shows the behavior of an oscillating cantileveras a function of a lever distance with respect to a surface. Thecantilever is externally driven to oscillate at a certain frequency. Theinteractions between the oscillating cantilever and the sample, inparticular, the sample surface, lead to a dependency of the oscillatingamplitude of the distance. This dependency can be evaluated to obtaininformation about the interactions. According to the invention, thedistance can be measured or calibrated on the basis of an additionalcantilever, the distance sensor.

Instead of an oscillation of the cantilever by external driving, one canalso use the thermal noise, i.e., thermal position fluctuations of thecantilever, to obtain information about the interaction between thecantilever and the sample surface.

FIG. 5, in diagram a), shows a frequency spectrum representing thermallyinduced vibrations of a cantilever at a distance 1000 nm of a surface.Diagram b) shows a corresponding positional autocorrelation function.Diagram c) shows a corresponding positional autocorrelation function fora distance of only 100 nm. From the measurement data, a number ofparameters characterizing the thermally induced vibrations of thecantilever may be calculated, for example, a resonance frequency and adamping coefficient.

Since thermal noise measurements require relatively long measurementperiods, it is very important to compensate any thermal drift tomaintain stable measurement conditions, in particular, with respect tothe distance. According to the invention, this can be effected by meansof a second cantilever, the distance sensor. Further, since there arechanges of the thermally induced cantilever oscillations on a minutedistance scale, it is very important to measure at well-defineddistances. According to the invention, this can be effected via theadditional cantilever, the distance sensor.

The high sensitivity of noise measurements for interactions between thecantilever and the surface and, possibly, a surrounding medium can beseen, in particular, in FIG. 6. Diagrams a) and b) show a noise spectrumand a corresponding positional autocorrelation function for a so-called“soft contact” of the cantilever tip on the surface, i.e., in this case,forces between the surface and the cantilever lower than 20 pN, whereasdiagrams c) and d) show the spectrum and the corresponding positionalautocorrelation function for a so-called “hard contact” between thecantilever tip and the surface, i.e., forces above 500 pN between thecantilever and the surface. If a cantilever probe measuring deviceaccording to the invention is used, stable measurement conditions (forexample, defined “soft contact” conditions) can be maintained even overa long measurement period, since at least one additional cantilever, thedistance sensor, allows a defined adjustment of the distance viafeedback control.

The invention provides a local probe measuring device for effectinglocal measurements referring to a sample, comprising a first local probefor local measurements with respect to a sample or a reference surface,a second local probe for local measurements with respect to the sampleor the reference surface, a measurement condition adjustment arrangementadapted to commonly adjust a first measurement condition of the firstlocal probe with respect to the sample or the reference surface and asecond measurement condition of the second local probe with respect tothe sample or the reference surface, a detection arrangement comprisinga first detection arrangement associated with the first local probeadapted to independently detect first measurement data referring tolocal measurements effected by said first local probe and a seconddetection arrangement associated with the second local probe adapted toindependently detect second measurement data referring to localmeasurements effected by said second local probe. Further, methods foreffecting local measurements and local manipulations by means ofmultiple local probes are provided.

1. A local probe measuring device for effecting local measurementsreferring to a sample, comprising: a first local probe for localmeasurements with respect to a sample or a reference surface, a secondlocal probe for local measurements with respect to the sample or thereference surface, a rigid mechanical coupling between the first localprobe and the second local probe, a positioning arrangement adapted tocommonly adjust distance relations of the probes with respect to thesample or the reference surface to commonly adjust a first measurementcondition of the first local probe with respect to the sample or thereference surface and a second measurement condition of the second localprobe with respect to the sample or the reference surface, and anoptical detection arrangement comprising a first detection arrangementassociated with the first local probe adapted to independently detectfirst measurement data referring to local measurements effected by saidfirst local probe and a second detection arrangement associated with thesecond local probe adapted to independently detect second measurementdata referring to local measurements effected by said second localprobe.
 2. The device according to claim 1, wherein said opticaldetection arrangement comprises at least one laser source and for atleast one of said local probes a respective position sensitive detectoradapted to receive a laser beam reflected by the respective local probe.3. The device according to claim 2, wherein a laser beam directed tosaid first local probe and the resulting reflected laser beam extend ina first plane and a laser beam directed to said second local probe andthe resulting reflected laser beam extend in a second plane, said firstand second planes arranged at an angle with respect to each other. 4.The device according to claim 3, wherein said angle is about 90°.
 5. Alocal probe measuring device for measuring local interactions between alocal probe arrangement and a sample, comprising: a first local probeadapted to interact locally with a sample or a reference surface, asecond local probe adapted to interact locally with the sample or areference surface, a rigid mechanical coupling between the first localprobe and the second local probe, a positioning arrangement adapted tocommonly adjust at least one of a first distance of the first localprobe with respect to the sample or the reference surface and a firstlocal interaction of the first probe with the sample or the referencesurface and at least one of a second distance of the second local probewith respect to the sample or the reference surface and a second localinteraction of the second local probe with the sample or the referencesurface, and a optical detection arrangement comprising a firstdetection arrangement associated with the first local probe adapted todetect at least one of the first distance and the first localinteraction independently of the second distance and the second localinteraction and a second detection arrangement associated with thesecond local probe adapted to detect at least one of the second distanceand the second local interaction independently of the first distance andthe first interaction.
 6. The device according to claim 5, wherein saidoptical detection arrangement comprises at least one laser source andfor at least one of said local probes a respective position sensitivedetector adapted to receive a laser beam reflected by the respectivelocal probe.
 7. The device according to claim 6, wherein a laser beamdirected to said first local probe and the resulting reflected laserbeam extend in a first plane and a laser beam directed to said secondlocal probe and the resulting reflected laser beam extend in a secondplane, said first and second planes arranged at an angle with respect toeach other.
 8. The device according to claim 7, wherein said angle isabout 90°.
 9. A local probe measuring device for measuring localinteractions between a cantilever probe arrangement and a sample,comprising: a first cantilever probe adapted to interact locally with asample or a reference surface, a second cantilever probe adapted tointeract locally with the sample or a reference surface, a rigidmechanical coupling between a base section of the first cantilever probeand a base section of the second cantilever probe, a positioningarrangement adapted to commonly adjust a first distance of the basesection of the first cantilever probe with respect to the sample or thereference surface and a second distance of the base section of thesecond local probe with respect to the sample or the reference surface,and an optical detection arrangement comprising a first detectionarrangement associated with the first cantilever probe adapted toindependently detect at least one of a first deflection of the firstcantilever probe and a first local interaction of the first cantileverprobe with said sample or reference surface and a second detectionarrangement associated with the second cantilever probe adapted toindependently detect at least one of a second deflection of the secondcantilever probe and a second local interaction of the second cantileverprobe with said sample or reference surface.
 10. The device according toclaim 9, wherein said optical detection arrangement comprises at leastone laser source and for at least one of said cantilever probes arespective position sensitive detector adapted to receive a laser beamreflected by a tip portion of the respective cantilever probe.
 11. Thedevice according to claim 10, wherein a laser beam directed to the tipportion of said first cantilever probe and the resulting reflected laserbeam extend in a first plane and a laser beam directed to the tipportion of said second cantilever probe and the resulting reflectedlaser beam extend in a second plane, said first and second planesarranged at an angle with respect to each other.
 12. The deviceaccording to claim 11, wherein said angle is about 90°.
 13. The deviceaccording to claim 1, wherein more than two local probes are providedfor local measurements with respect to the sample or the referencesurface, which are rigidly mechanically coupled with each other and arecommonly adjustable by the positioning arrangement.
 14. The deviceaccording to claim 13, wherein said optical detection arrangement isadapted to independently detect measurement data referring to localmeasurements effected by each local probe.
 15. The device according toclaim 5, wherein more than two local probes adapted to interact locallywith the sample or the reference surface are provided, which are rigidlymechanically coupled with each other and are commonly adjustable by thepositioning arrangement.
 16. The device according to claim 15, whereinsaid optical detection arrangement is adapted to independently detectthe distance of each local probe with respect to the sample or thereference surface or the interaction of each local probe with the sampleor the reference surface.
 17. The device according to claim 9, whereinmore than two cantilever probes adapted to interact locally with thesample or the reference surface are provided, which are rigidlymechanically coupled with each other at a respective cantilever baseportion and are commonly adjustable by the positioning arrangement. 18.The device according to claim 17, wherein said optical detectionarrangement is adapted to independently detect a respective deflectionor interaction of each cantilever probe.